HD 191939: Three Sub-Neptunes Transiting a Sun-like Star Only 54 pc Away
Mariona Badenas-Agusti, Maximilian N. Günther, Tansu Daylan, Mikal-Evans, Thomas, Chelsea X. Huang, Elisabeth C. Matthews, Benjamin V. Rackham, Rackham, Benjamin V., Sara Seager, G. Ricker, R. Vanderspek, Prajwal Niraula, Ian J. M. Crossfield, Michael Fausnaugh, G. Nowak, I. Ribas, Ε. Πάλλη, Jon M. Jenkins, David W. Latham, Seager, Sara, Ricker, George R., Vanderspek, Roland K., Joshua N. Winn, Abril-Pla, Oriol, Karen A. Collins, Serra, Pere Guerra, Niraula, Prajwal, Zafar Rustamkulov, Thomas Barclay, Crossfield, Ian J. M., Steve B. Howell, David R. Ciardi, Erica J. Gonzales, Joshua E. Schlieder, Douglas A. Caldwell, Fausnaugh, Michael, Scott McDermott, Martin Paegert, Joshua Pepper, Mark E. Rose, Joseph D. Twicken
Abstract
We present the discovery of three sub-Neptune-sized planets transiting the nearby and bright Sun-like star HD 191939 (TIC 269701147, TOI 1339), a K s = 7.18 mag G8 V dwarf at a distance of only 54 pc. We validate the planetary nature of the transit signals by combining 5 months of data from the Transiting Exoplanet Survey Satellite with follow-up ground-based photometry, archival optical images, radial velocities, and high angular resolution observations. The three sub-Neptunes have similar radii (Rb=3.42-0.11+0.11, Rc=3.23-0.11+0.11, and Rd=3.16-0.11+0.11,R⊕), and their orbits are consistent with a stable, circular, and coplanar architecture near mean-motion resonances of 1:3 and 3:4 (P b = 8.88, P c = 28.58, and P d = 38.35 days). The HD 191939 system is an excellent candidate for precise mass determinations of the planets with high-resolution spectroscopy due to the host star's brightness and low chromospheric activity. Moreover, the system's compact and near-resonant nature can provide an independent way to measure planetary masses via transit timing variations while also enabling dynamical and evolutionary studies. Finally, as a promising target for multiwavelength transmission spectroscopy of all three planets' atmospheres, HD 191939 can offer valuable insight into multiple sub-Neptunes born from a protoplanetary disk that may have resembled that of the early Sun.